1. Field of the Invention
The present invention relates to a coil unit for generating magnetic fields exhibiting desired spatial distribution. More particularly, this invention is concerned with an epoch-making way of winding a coil to be included in a magnetic-field generation coil unit preferably of a magnetic resonance imaging (MRI) system or magnetic resonance spectroscopy (MRS) system that utilizes a resonance phenomenon exhibited by nuclear spins of a subject.
2. Description of the Related Art
A coil for generating a magnetic field is an indispensable element for many electric circuits or electric equipment. The gantry of a medical-purpose magnetic resonance imaging (MRI) system or magnetic resonance spectroscopy (MRS) system is one such system. A static coil for generating a static magnetic field, shim coils used to compensate for inhomogeneities in the static magnetic field, gradient coils for generating a magnetic field gradient to be superposed on the static magnetic field, and a radio-frequency coil used to transmit or receive radio-frequency signals are used as magnetic-field generation coils.
These coils employed in an MRI or MRS system are, unlike an inductive element in an ordinary electric circuit, requested to meet another requirement that they must generate magnetic fields exhibiting a spatially desired distribution (also having a desired magnetic field strength). In particular, the gradient coils to which a pulsating current is fed are supposed to meet requirements defining switching characteristics such as the rise time required until a maximum magnetic field gradient strength is attained.
A space in which a subject and the radio-frequency coil are inserted must be preserved inside the gantry of the MRI or MRS system. Various coils are arranged around the space. The gantry itself therefore tends to get large in size. Currently, it is sought to improve the ability to generate magnetic fields while avoiding an increase in size. A coil layer containing the static coil, shim coils, and gradient coils must be wound as thinly as possible. Especially, in the case of gradient coils to be stored in an already-defined-size bore of the static coil (for example, a superconducting magnet), the coil layer must be wound in a layer.
With respect to the gradient coils, shielded coils capable of preventing magnetic leakage have been widely adopted in recent years. One of the shielded gradient coils is an actively (self-) shielded gradient coil (ASGC). This coil assembly has a dual coil structure having a main coil enclosed with a shield coil. It is therefore required that the main coil and shield coil are each wound in a layer in order to realize a thin coil assembly.
As far as a coil assembly employed in an MRI or MRS system is concerned, the positions of windings forming a coil must be determined so that a spatially desired distribution of magnetic fields can be attained. As a known method of designing a coil, a technique using a continuous distribution or function to design a coil that exhibits a desired distribution of magnetic field is well-known. Also known is a technique described in "Gradient Coil Design: A Review of Methods" written by R. Turner (Magnetic Resonance Imaging, Vol. 11, pp.903-920, 1993). According to Turner's proposal, "integrated currents (amp-turns)" are calculated by integrating a distribution of current densities (See FIG. 7(A) in p. 911 in the same thesis). Spatial positions associated with the integrated values of current densities are defined as coil positions by increasing the same value on a curve indicating the integrated values. This technique is called "target field approach in Turner's proposal.
Still, as a conventional coil arrangement design technique, a distribution of current densities is used to determine the positions of windings, which is described, for example, in "Designing an NMR Actively Shielded Gradient Coil" written by Kiyoshi Yoda (T. IEE Japan, Vol. 110-A, No. 4, p.275-281, 1990), is known. This technique is such that a desired distribution of current densities is calculated for an axial distance on a cylindrical bobbin of a coil, the distribution is integrated sequentially from the axial center of the bobbin toward each axial end thereof, and the positions of windings (turns) are determined from the axial center toward each axial end by examining axial points where the integrated values become I/2, I, . . . , I,I/2 (I: coil drive current value).
Thus, in the known method proposed by Yoda, an ideal continuous distribution of currents obtained analytically is replaced with a discontinuous distribution of currents externalized as windings (changed into a discrete distribution) in order to create a coil assembly having a wire wound in a layer.
However, in the foregoing method of designing a coil, since an ideal continuous distribution obtained analytically is replaced with discontinuous coil positions, an error occurring in an actual distribution of currents is basically unavoidable. For this reason, a desired ideal distribution of magnetic fields cannot be attained in many cases. Taking a gradient coil for instance, the linearities of magnetic field gradients realized with the continuous distribution deteriorate. Even the static coil and shim coils are designed according to the foregoing technique. For the same reason, there arises a problem that a distribution of magnetic fields deviates from a desired ideal state, and the homogeneities in a static magnetic field cannot be attained as expected. When an actual distribution of magnetic fields deviates from an ideal state, an adverse effect imposed on the qualities of MR images becomes serious, and the reliabilities of the images are impaired. From this viewpoint, there is a need for attaining a desired ideal distribution of magnetic fields.
In addition to basic problems concerning the change from continuity to discontinuity for attaining a discrete distribution, there are problems in the arrangement of coil windings proposed by A Yoda, which are described below.
A Yoda-proposed arrangement design needs a prerequisite that the design is carried out under strict restrictions including a condition where the peak value of a streamline function curve calculated based on a distribution of current densities supplied to a shield coil of Z channel is exactly an integer-times the coil drive current I. In the actual design, however, it will hardly happen that a solution to meet such condition will have found, in most cases, a remaining current which cannot covered by the windings being left. The remaining current thus appears at and have influence on the axial ends of a shield coil, because the positions of windings are determined from the axial center of the shield coil to the axial ends thereof. In consequence, at the axial end portions of a shield coil is provided a vacant gap which is relatively large and has no turns. Magnetic fluxes will leak because of the gap, causing eddy currents to flow on and in surrounding metal frames. In particular, the eddy currents thus-caused at the axial end portions have unfavorable deteriorating effects on the quality of MR images. On one hand, analytically designing the positions of windings of a coil based on Yoda's proposal should sacrifice its performance such as linearity. Additionally, the inductance and resistance values of the coil becomes large in Yoda's proposal, resulting in a larger-sized (i.e. enhanced power output) gradient coil. The coil design by Yoda's proposal is thus faced with various difficulties for practical use.
Still, in addition to the basic problem in changing into discrete winding positions described above, the conventional coil design techniques including the foregoing Yoda's proposal have problems as below.
First, there is a problem of physical restrictions to be imposed on an arrangement of windings forming a coil. Assuming that a coil having a wire is created by winding a wire, which has a certain width, about a cylindrical bobbin, the actual width of the coil is determined with the width over windings of the area most crowded with windings (turns) (area in which windings are most dense). In other words, there is the restriction that a wire wider than the width over windings of an area most crowded with windings cannot be used. Because of this restriction, when an ideal continuous distribution of currents is replaced with a discontinuous distribution of currents, a wide gap in which no wire exists is created between windings of a coil.
The gap between windings poses a serious problem on, especially, an actively shielded gradient coil. Whether a gradient coil is of a saddle type or solenoid type, the size of the gap varies depending on the position in a coil unit. A wide gap between windings allows magnetic fluxes to leak out. As a result, eddy currents are induced in an external conductor. Despite the actively shielded gradient coil, the magnetic fields affected by the eddy currents invite deterioration of qualities of MR images. This has become a serious problem in recent years.
The above situation will be described further. With the advancements of various electronic technologies and superconducting technologies, echo planar imaging (EPI) is one fast imaging technique enabling fast imaging that is faster than known spin echo (SE) imaging and fast spin echo (FAST SE) imaging and it has come to be a mainstream imaging technique in recent years. Spin echo imaging requires certain performance in relation to magnetic field gradients, for example, a maximum magnetic field gradient strength of 10 mT/m and a rise time of 1 msec required until the maximum magnetic field gradient strength is attained. By contrast, echo planar imaging requires certain performance in relation to magnetic field gradients, for example, a maximum magnetic field gradient strength of 30 mT/m and a rise time of 0.1 msec required until the maximum magnetic field gradient strength is attained.
With such increases in maximum magnetic field gradient strength and decreases in rise time, magnetic leakage increases. The increase in magnetic leakage brings about various deteriorations in image quality. This problem has become especially significant in recent years.
Even when the structure of an ASGC is adopted, the problem that eddy currents are induced is pointed out even in "Design and Evaluation of Shielded Gradient Coils" written by J. W. Carlson et al. (Magnetic Resonance Imaging, Vol. 26, pp.191-206, 1992). As for the problem that eddy currents are induced, various countermeasures have been proposed. The problem of eddy currents is solved by improving a pulse sequence used to acquire an MR signal or by optimizing the phase of an radio-frequency pulse to be applied.
However, when any of such proposed countermeasures is adopted, sequence control becomes complex. Besides, the practical efficacy is very low. The reasons are as follows: if magnetic fields affected by eddy currents exhibit the same spatial distribution (are the same magnetic field components) as magnetic field gradients, correction through pulse sequence control can be achieved. However, in reality, almost all magnetic fields affected by eddy currents contain magnetic components different from magnetic field gradients. It is, in principle, impossible to correct such magnetic fields affected by eddy currents on an ex post facto basis by controlling magnetic field gradients, a radio-frequency pulse, and a pulse sequence. In short, there is no better measure other than suppressing induction of eddy currents themselves. At present, eddy currents resultant from a gap between shielding windings included in a self-shielded gradient coil are thought to be unavoidable. This problem has remained unsolved.
For smoothing a distribution of actually generated magnetic fields and approximating it to a desired distribution of magnetic fields, it is thought that a markedly thin wire is used in order to increase the number of turns. However, such a coil has a markedly high resistance and inductance. Unless the current-carrying capacity of a power supply increases enormously, a current cannot be supplied to the coil. It is actually very hard to manufacture such a large-capacity power supply. At present, it is rather unfeasible to manufacture such a large-capacity power supply.